Histotripsy for thrombolysis

ABSTRACT

Methods for performing non-invasive thrombolysis with ultrasound using, in some embodiments, one or more ultrasound transducers to focus or place a high intensity ultrasound beam onto a blood clot (thrombus) or other vascular inclusion or occlusion (e.g., clot in the dialysis graft, deep vein thrombosis, superficial vein thrombosis, arterial embolus, bypass graft thrombosis or embolization, pulmonary embolus) which would be ablated (eroded, mechanically fractionated, liquefied, or dissolved) by ultrasound energy. The process can employ one or more mechanisms, such as of cavitational, sonochemical, mechanical fractionation, or thermal processes depending on the acoustic parameters selected. This general process, including the examples of application set forth herein, is henceforth referred to as “Thrombolysis.”

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 16/293,394, filed Mar. 5, 2019, now U.S. Pat. No. 11,364,042, which is a continuation of U.S. patent application Ser. No. 12/358,549, filed Jan. 23, 2009, now U.S. Pat. No. 10,219,815, which is a continuation-in-part of U.S. patent application Ser. No. 12/121,001, filed May 15, 2008, now U.S. Pat. No. 8,057,408, which is a continuation-in-part of U.S. patent application Ser. No. 11/523,201 filed Sep. 19, 2006, now abandoned, which claims the benefit of U.S. Provisional Patent Application No. 60/786,322, filed Mar. 27, 2006, U.S. Provisional Patent Application No. 60/719,703, filed Sep. 22, 2005, and U.S. Provisional Patent Application No. 60/753,376, filed Dec. 22, 2005.

U.S. patent application Ser. No. 12/121,001 filed May 15, 2008, further claims the benefit of U.S. Provisional Patent Application No. 60/938,806, filed May 18, 2007.

U.S. patent application Ser. No. 12/358,549, filed Jan. 23, 2009, further claims the benefit of U.S. Provisional Application No. 61/023,554 filed Jan. 25, 2008. The entire disclosure of each of the above applications is incorporated herein by reference.

GOVERNMENT INTEREST

This invention was made with government support under Grant No. EB008998 awarded by the National Institutes of Health. The government has certain rights in the invention.

FIELD

The present teachings relate to ultrasound therapy and, more particularly, relate to methods and apparatus for performing “thrombolysis,” as defined herein, in a safe, effective, noninvasive manner using direct image guidance.

BACKGROUND

This section provides background information related to the present disclosure which is not necessarily prior art.

Thrombosis is the medical term for the process of pathologic blood clot formation—the key mechanism underlying many cardiovascular diseases, including stroke, myocardial infarction, deep vein thrombosis (DVT), etc. These thrombi can break off from site of formation and travel to distant sites (embolisation) and cause symptoms at sites distinct from the site of formation. Further these processes may manifest in conduits that are placed in the vascular bed to bypass blood flow (e.g., grafts) or as extensions to the vascular bed (e.g., drive lines for cardiac assist devices, implantable venous catheters, etc.). Each of these conditions poses a significant clinical problem. For example, DVT is the formation of blood clots in the deep veins, most commonly those of the lower legs. DVT has an incidence rate of 1 in 1000 persons. Up to 5% of patients with DVT experience pulmonary embolism, which causes at least 100,000 deaths annually in the United States.

To treat thrombosis, the pathologic blood clot (thrombus) or clot fragment (embolus) needs to be removed. Current clinical treatments to remove thrombi include thrombolytic drugs, catheter-based surgical procedures, and direct surgical removal of clots. Treatment of thrombosis usually encompasses either breakup of the clot (thrombolysis) or removal (thrombectomy). These terms are used in reference to both thrombus and emboli irrespective of site of formation or disease and are used herein as such.

Thrombolytic drugs (e.g., rt-PA) dissolve the blood clot by breaking down the cross-linked fibrin structures that solidify the clot. Thrombolytic drugs systemically stimulate the fibrinolytic process while suppressing the anti-fibrinolytic process. Therefore, both thrombosis and normal hemostatic clot formation (vessel wound healing) are inhibited. Inhibition of normal hemostatic clot formation is associated with an increase in bleeding complications, which may be fatal in a small number of cases.

In contrast, treatments using catheter-based devices are localized to the target clot. The current catheter-based thrombolysis procedures include catheter-based local delivery of thrombolytic agents, vein segment isolation and thrombolysis, and mechanical disruption and aspiration of the clot (Rheolytic thrombectomy). However, catheter-based devices are invasive and carry an increased risk of bleeding, damage to the vessel wall, and infection. In rare cases, catheter-based thrombolysis methods may also result in death.

Direct surgical methods are even more invasive than catheter-based methods. Clinicians make a small incision through the skin and surgically remove the clot directly.

Researchers have been exploring new means to improve the efficiency and safety of thrombosis treatment techniques. Minimally invasive or non-invasive ultrasound methods to treat thrombosis have been developed.

Studies have shown that ultrasound energy can accelerate thrombolysis by facilitating the delivery of thrombolytic drugs to the target clot. Thrombolysis refers to dissolving or breaking up of a thrombus. For example, ultrasound combined with rt-PA can dissolve a clot within 30 minutes, which would otherwise take 3 hours using rt-PA alone. Ultrasound energy can be generated by inside the vessel through a catheter-based transducer (Rosenschein et al., U.S. Pat. No. 5,163,421, Tachibana et al., U.S. Pat. No. 6,001,069) or outside the patient body through an external transducer non-invasively (Holland et al., U.S. Pat. No. 7,300,414). Even though this method increases thrombolysis efficiency, it still carries the undesired side effects of thrombolytic drugs. This hybrid technique is still being studied and not currently in clinical use.

Recently, some researchers have been investigating the possibility of achieving thrombolysis using ultrasound alone or combined with contrast agents, without the use of pharmaceutical drugs. Using microbubbles induced by high intensity focused ultrasound (Rosenschein et al. U.S. Pat. Nos. 5,524,620 and 6,113,558) or via injected contrast agents (Unger et al., U.S. Pat. No. 6,576,220, Siegel et al., U.S. Pat. No. 5,695,460), blood clot removal can be achieved. Similarly, ultrasound energy can be generated inside the vessel or outside the patient body. However, the mechanism is not well understood, and therefore, these techniques remain far from clinical application.

Acoustic cavitation has been claimed to be a possible mechanism of some older ultrasound thrombolysis methods. Acoustic cavitation is a term used to define the interaction of an acoustic field, such as an ultrasound field, with bodies containing gas and/or vapor. This term is used in reference to the production of small cavities, or microbubbles, in the liquid. Specifically, when an acoustic field is propagated into a fluid, the stress induced by the negative pressure produced can cause the liquid to rupture, forming a void in the fluid which may contain vapor and/or gas. Acoustic cavitation also refers to the oscillation and/or collapse of microbubbles in response to the applied stress of the acoustic field. However, no one has previously succeeded in achieving controlled and predictable cavitation for thrombolysis with real-time ultrasound feedback.

Methods have been developed to initiate, maintain, and control cavitation for use in general therapy. For example, Cain et al. (U.S. Pat. No. 6,309,355), which is hereby incorporated by reference, describes apparatus and methods that use cavitation induced by an ultrasound beam to create a controlled surgical lesion in a selected therapy volume of a patient.

As indicated, previous ultrasound thrombolysis methods involve the use of thrombolytic drugs or microbubbles. Other methods that use ultrasound energy alone, invasive methods or even noninvasive methods, do not allow easy assessment or feedback of when the process is operating effectively, and often do not provide any feedback which can be used to optimize the process. Consequently, more effective methods and techniques for ultrasound thrombolysis therapies are desirable and would enable beneficial noninvasive alternatives to many present methods in the thrombosis treatment field. In particular, monitoring treatment and receiving feedback during the procedure would inform a clinician whether the procedure is progressing adequately according to plan and when it can be ended. As such, the ability to monitor and adjust the ultrasound thrombolysis therapy concomitant with treatment would provide significant advantages over prior ultrasound thrombolysis therapies.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

According to the principles of the present teachings, novel ultrasound devices and methods for performing non-invasive thrombolysis with ultrasound are provided. Briefly, the method uses one or more ultrasound transducers to focus or place a high intensity ultrasound beam onto a blood clot or other vascular inclusion or occlusion (e.g., clot in the dialysis graft, deep vein thrombosis, superficial vein thrombosis, arterial embolus, bypass graft thrombosis or embolization, pulmonary embolus) which would be ablated (eroded, mechanically fractionated, liquefied, or dissolved) by ultrasound energy. The process can employ one or more mechanisms, such as of cavitational, sonochemical, mechanical fractionation, or thermal processes depending on the acoustic parameters selected. This general process, including the examples of application set forth herein, is henceforth referred to as “Thrombolysis.”

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 illustrates an experimental apparatus for in-vitro thrombolysis. A blood clot is placed in an LDPE tube and the therapy transducer aligned with the focus at one end of the clot using a 3-axis positioning system. An ultrasound imager is located concentric with the therapy transducer for image-guidance during treatment. A 5 MHz single-element transducer to record backscatter was mounted perpendicular to the therapy transducer with their foci overlapping (not shown). The dashed lines show the connection of the circulatory flow system, when present. In static saline, the ends of the tube are plugged with rubber stoppers.

FIG. 2 illustrates pressure waveforms of therapy pulses at the focus of the transducer. The signals shown are averages of 200 pulses. The peak negative pressure is listed above each waveform. Measurements were recorded using a fiber optic probe hydrophone.

FIG. 3 illustrates progression of treatment in static saline. Ultrasound propagation is from right to left in the image. The clot moves into the focus of the transducer almost immediately after ultrasound exposure is started generated. The clot quickly loses mass and is bisected at the focus. Each of the two larger pieces is then dissolved over 45 seconds until no visible particles remain.

FIGS. 4(A) and 4(B) illustrate thrombolysis rate as a function of peak negative pressure at the therapy focus (mean+/−standard deviation, n=8). Pressures below 6 MPa had no observable effect on the clot after 5 minutes of treatment. At 6 MPa or greater, an increase in rate is observed, and the clot is quickly dissolved in times ranging between 80-300 seconds. (B) Percentage of initiated time versus peak negative pressure. The percentage of initiated time is defined as the initiated time divided by the total treatment time. Initiation here refers to the initiation of a temporally changing backscatter described in the text. For pressures <6 MPa, initiation was never detected. Above 6 MPa, initiation was always observed and the initiated state remained throughout the treatment.

FIGS. 5(A)-5(C) illustrate B-Mode images of the histotripsy thrombolysis treatment using a 5 MHz imaging probe. The imaging probe is approximately 8 cm from the ultrasound focus. The ultrasound propagation is from top to bottom of the image. The clot is visible in the tube as an echogenic region prior to insonation (A). The bubble cloud is visible during treatment in (B). The vertical lines in (B) are acoustic interference of the therapy transducer with the imager. However, most of the image remains uncorrupted. The echogenicity of the clot is greatly reduced after complete thrombolysis (C).

FIG. 6 illustrates debris volume distribution by particle diameter as measured by the Coulter Counter. A majority of the debris volume is smaller than 10 μm diameter for samples at all pressure levels. However, an increase in larger particles (30-60 μm) is apparent at 10 and 12 MPa. The number of measurements where the 100 μm tube was blocked (number of particles >60 um) is listed above each bar in the figure. There were 16 measurements taken at each pressure level.

FIG. 7 illustrates progression of an experiment demonstrating the NET technique. Ultrasound propagation is from top to bottom. A clot fragment flows into the bubble cloud at the focus of the transducer generated prior to arrival of the fragment, with p− of 12 MPa. The clot fragment remains near the cloud at the transducer focus, and is further broken down over the course of 60 seconds. The bubble cloud is transparent and not visible in the images. The mean background flow rate is −5 cm/s from left to right.

FIGS. 8(A)-(D) illustrate histological slides (H&E stain) from treatment of clots in canine inferior vena cava segments. A control sample is shown in (A) and a magnified view in (B). A treated sample exposed to 300 seconds of ultrasound at p− of 12 MPa is shown in (C) and a magnified view (D). Both samples were intact, and no discernable damage was observed to the treated vein wall.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference to the accompanying drawings.

Thrombosis is the formation of a blood clot in vasculature and is the primary cause of many vascular diseases, including heart attack, stroke, pulmonary embolism (PE) and deep vein thrombosis (DVT). Current clinical methods to treat thrombosis include anticoagulant and thrombolytic drugs, catheter-based surgical techniques, or a combination of the two where a catheter is used to locally deliver the thrombolytic agent to the site of occlusion. Thrombolytic drugs (e.g., Streptokinase, urokinase, rt-PA etc.) administered without a catheter require long treatment times (several hours) and are non-specific, with a substantial risk of major bleeding that can be fatal in a small number of cases. The current catheter-based thrombolysis procedures include local delivery of thrombolytic agents by catheter, vein segment isolation and thrombolysis, and mechanical disruption and aspiration of the clot (rheolytic thrombectomy). Catheter-based devices have the advantage of localizing treatment to the clot, but are invasive and also carry an increased risk of hemorrhage, damage to the vessel wall, and infection. Surgical procedures also increase the cost of treatment due to additional patient care and monitoring post-operationally.

Ultrasound has been known for several decades to promote clot breakdown, as both a stand-alone procedure and used in conjunction with thrombolytic drugs or contrast agents. Many groups have reported an increase in thrombolytic efficiency of rt-PA and streptokinase when low-intensity, non-focused ultrasound was applied. A reduction in average lysis time from 3 hours to 30 minutes has been achieved for combined ultrasound+rt-PA therapy compared with just rt-PA alone. However, these methods still carry the risks of major bleeding associated with thrombolytic drugs. Alternatively, ultrasound has also been used by itself or in conjunction with catheters to locally administer thrombolysis. While catheter-based methods can quickly disrupt the occlusion, they also have the drawbacks associated with surgical techniques and may cause damage to the surrounding vessel. In-vitro studies have shown high-intensity focused ultrasound operated in a pulsed-mode induces rapid clot breakdown without thrombolytic drugs. Westermark et al. and Rosenschein et al. found that pulsing a focused transducer was more effective than either continuous-wave high-intensity ultrasound or lithotripsy shockwaves. The increased efficacy was attributed to activity of cavitation induced by the pulsing regime. The underlying mechanisms of cavitation damage, however, remain poorly understood.

In connection with the present disclosure, a new non-invasive thrombolysis method—histotripsy, which uses pulsed ultrasound alone—was investigated. This technology depends on control of cavitation to mechanically fractionate cells and tissues using focused ultrasound pulses. This technique can be viewed as soft tissue lithotripsy, giving rise to the name “histotripsy”. The pulsed cavitational therapy process is similar to lithotripsy in that soft tissues are progressively mechanically subdivided instead of hard kidney stones. The present process of pulsed cavitational ultrasound is also referred to herein as histotripsy, connoting essentially the lithotripsy of soft tissues. The histotripsy process of the present teachings can, at least in part, involve the creation and maintenance of ensembles of microbubbles in the form of a bubble cloud and, in some embodiments, the use of feedback in order to optimize the process based on observed spatial-temporal bubble cloud dynamics.

It was found that cavitation nucleation can be controlled to create targeted tissue fractionation using appropriate ultrasound pulse sequences assisted by cavitation-based feedback monitoring. Histotripsy pulses include successive, very short (<50 cycles), high-pressure (>6 MPa) nonlinear pulses delivered at low duty cycles (0.1-5%). Cavitation can be monitored using acoustic feedback such as ultrasound backscatter.

It has been found that histotripsy can fractionate soft tissue to acellular debris within a few minutes. Histotripsy can be visualized and guided using real-time ultrasound imaging. The bubble cloud generated by histotripsy is visible as a highly-dynamic echogenic region on a B-Mode image, allowing precise targeting prior to treatment. The fractionated tissue shows a reduction in echogenicity compared with intact tissue, which can be used to evaluate progression of treatment. In vascular systems, Doppler ultrasound can also provide feedback and confirm restoration of flow after thrombolysis. The abilities to efficiently fractionate tissue and monitor therapy using image-guided real-time feedback are primary motivations to explore histotripsy as a potential non-invasive thrombolysis method.

Histotripsy thrombolysis method contains three general steps, all of which are guided by real-time imaging. First, the clot is targeted by the therapy focus prior to the treatment. Histotripsy pulses are used to create a bubble cloud without the presence of the clot, which appears on the image and is marked as the therapy focus. Targeting is achieved by moving the transducer to align the focus to the clot. Second, histotripsy treatment is applied using appropriate initiating and sustaining therapy ultrasound sequences. The treatment progress is monitored by detecting the bubble cloud, the clot and the blood flow in the vessel. Third, the treatment completion is determined by imaging the vessel and blood flow in the vessel.

A key part of the histotripsy process is that each incident ultrasound pulse has two primary functions. First, it produces a small fraction of the desired therapy result. Second, it predisposes the target volume to effective tissue interaction for the next pulse. A set of parameters, including but not limited to intensity, peak negative pressure, peak positive pressure, time of arrival, duration, and frequency, thus allows for many feedback, optimization, and real time monitoring opportunities.

Once initiated, each pulse produces a bubble cloud, or set of cavitationally active microbubbles, that, as indicated herein, produces part of the tissue therapy and produces microbubbles predisposing the volume to subsequent pulses. After initiation the process can progress with assurance that each pulse effectively participates in the therapy process. Each individual pulse breaks down a small portion of the thrombus and many pulses, from several thousand to over a million, are required to completely break down the whole clot.

Since each pulse produces a bubble cloud, it can be easily seen by ultrasound imaging scanners or by special transducers used to detect the ultrasound backscatter. In the case of the imaging systems, the bubbles show up as a bright spot on the image that can be localized to the desired place on the image by moving the therapy transducer focus either mechanically or via phased array electronic focus scanning. Because of the very short pulses (5 μs) and very low duty cycle (0.1%-5%), only a small number of ultrasound B-scan lines are corrupted by the histotripsy pulse interference. By comparison, most ultrasound therapy methods use long pulses (100 msec or longer) or continuous waves and ultrasound imaging during treatment is often completely corrupted by the interference of therapeutic ultrasound.

Pulsed cavitational ultrasound therapy, or the histotripsy process according to the present teachings, can include four sub-processes, namely: initiation, maintenance, therapy, and feedback, which are described in detail herein.

During the initiation step, cavitation nuclei are generated, placed, or seeded in the therapy volume, which is the portion of tissue to which the therapy is directed. The cavitation nuclei reduce the threshold for cavitation by subsequent therapy pulses. Without initiation, the therapy process will not proceed with typical therapy pulses. Initiation assures that the process will progress until it spontaneously (or through active intervention) extinguishes.

During the maintenance step, the presence of micro-nuclei in the therapy volume is actively maintained, assuring that subsequent therapy pulses will produce the appropriate tissue effect, breakdown of the thrombus in this disclosure. During the therapy step, the micro-nuclei (likely small microbubbles) that have been properly initiated and maintained by the preceding processes can be impinged upon by a therapy pulse that produces acute cavitation and tissue fractionation. Each therapy pulse can produce just a small part of the overall mechanical fractionation.

In the simplest process, the therapy transducer initiates, maintains, and produces the desired therapy effect. Thus, for example, a series of high intensity pulses are focused onto the therapy volume sufficient to initiate the bubble clouds. The intensity of the pulses can then be decreased to an intermediate intensity that is below a value that would not otherwise initiate the process. This intermediate intensity is sufficient to sustain the process, otherwise, the process can be re-initiated, if necessary, to produce adequate tissue fractionation. As will be described herein, feedback on the bubble cloud presence or absence can be obtained by monitoring the therapy pulse backscatter from the bubble cloud, where backscatter absence indicates an extinguished process. The backscatter is monitored by the therapy transducer (or subset of therapy transducer array elements) in the receive mode, or by a simple (and separate) monitoring transducer. In some embodiments, multiple transducers can be employed for monitoring feedback.

During the feedback step, each of the prior sub-processes can be monitored to thereby monitor overall therapy progression. The feedback and monitoring step allows for various parameters of the pulsed cavitational ultrasound process to be varied in real time or in stages, if desired, permitting controlled administration of the ultrasound therapy. For example, the process can be terminated, the extent of therapy measured, and the process reinitiated. In particular, the feedback sub-process enables adjustment and tuning of the histotripsy process in precise and controlled ways previously unobtainable.

It should be noted that methods of the present teachings can include variations where each of these four sub-processes can use different methods of energy delivery with different forms of energy and different feedback schemes. Additional details of various embodiments of each subprocess follow.

A. Initiation:

Initiation can comprise an initiation pulse sequence, which is also referred to herein as an initiation sequence or pulse, or initiation. Initiation introduces cavitation threshold-reducing cavitation nuclei and can be accomplished with a therapy transducer using acoustic energy, usually high intensity pulses, at the same frequency as the sustaining and therapy processes. However, initiation can be accomplished by other forms of energy including high intensity laser (or optical) pulses that create a vapor cloud or even a plasma cloud, or x-rays (the ionizing radiation bubble chamber effect). Cavitation nuclei can also be injected intravascularly, or can be injected, or shot (mechanically jetted) into the therapy volume. Thermal means can also be employed wherein elevated temperature, e.g., via a laser, can introduce vapor nuclei (boiling for example). Microbubbles (or proto-bubble droplets, e.g., perfluorocarbon droplets) can be targeted to a therapy volume by molecular or other recognition mechanisms, e.g., antibody against tumor antigens conjugated to nuclei (or proto-nuclei) that would concentrate in or near a tumor. Targeted substances can also be more general than microbubbles or proto-nuclei, such as enzymes, proteins, or other molecules or constructs that enhance the enucleation (gas bubble generation) of dissolved gas into actual microbubbles. Initiation can also occur via mechanical stimulation sufficient to generate cavitation or cavitation nuclei. Initiation, in some embodiments, can be accomplished by an ultrasound imaging transducer whose other role is obtaining feedback information on the histotripsy process or feedback on the therapy itself.

An effective acoustic approach is to use a separate acoustic transducer(s), which can be an array or a plurality of transducers, to initiate, and then use the therapy transducer for the maintenance and therapy sub-processes. This would enable one to use high frequency ultrasound for initiation thus making use of the higher resolution of high frequency transducers or arrays. In this embodiment, initiation could aid in determining the outlines of the therapy volume with high spatial resolution. Therapy could then progress at lower frequencies using the therapy transducer or an array of transducers. For example, lower frequencies would propagate through some bone and air. Thus, methods can include predisposing (initiating) with high resolution and disposing (providing therapy) at a lower frequency that can cover the entire therapy volume. Lower frequency sound propagates more easily through bone and air, enabling methods of the present teachings to be applied to sites beyond such structures. In addition, lower frequency sound has lower thermal absorption, reducing heat generation.

Feedback is important in determining if initiation has occurred because the therapy process will not progress without initiation. In some embodiments, feedback can include monitoring the backscattered signal from the therapy pulses. If no significant backscatter occurs, initiation has not been successful or the process has extinguished and needs to be re-initiated. In some embodiments, feedback can employ one or more of the following: an ultrasound imaging modality that would detect the microbubbles as a hyperechoic zone; a separate transducer to ping (send an interrogation pulse or pulses) and a transducer to receive it; optical processes wherein optical scattering from the microbubbles (when initiated) is detected; MRI imaging to detect the microbubbles; and low frequency hydrophones, which can detect the low frequency sound produced when bubble clouds expand and contract.

In some embodiments, the feedback scheme can determine the parameters of the existing cavitation nuclei and their dynamic changes with sufficient precision to predict the optimum characteristics or parameters for the next therapy pulse (intensity, peak negative pressure, peak positive pressure, time of arrival, duration, frequency, etc.).

B. Maintenance:

Maintenance can comprise a sustaining pulse sequence, which is also referred to herein as a sustaining sequence, sustaining or maintenance pulse, or maintenance. Maintenance can follow initiation and can also be part of initiation. Generally, once initiated, the cavitation process must be maintained or it will spontaneously extinguish. For example, cavitation can be extinguished when the next therapy pulse does not generate another bubble cloud or does not encounter sufficient nuclei to effectively cavitate at least a portion of the therapy volume. In various embodiments, maintenance is accomplished by the next therapy pulse that creates a bubble cloud that leaves behind sufficient nuclei for the following pulse.

Maintenance can also be accomplished by a separate sustaining transducer producing ultrasound to maintain (sustain) the appropriate nuclei characteristics and population. Thus, the separate transducer(s) described herein for initiation can also maintain (sustain) the nuclei. Likewise, in some embodiments, maintenance can be continued by optical means, x-rays (ionizing radiation), mechanical stimulation, or thermal means. In some embodiments, maintenance can be accomplished by a feedback ultrasound imaging transducer. For example, if a slow therapy pulse repetition frequency is desired (e.g., to prevent tissue heating), sustaining sequences or pulses (of lower intensity, for example) can be interleaved between the therapy pulses to sustain the microbubble or nuclei population and characteristics necessary to allow the next therapy pulse to be effective. These interleaved sustaining sequences can be applied by the various means enumerated herein for maintenance or initiation.

C. Therapy:

Therapy can comprise a therapy pulse sequence, which is also referred to herein as a therapy sequence, therapy pulse, or therapy. The therapy process is the interaction of ultrasound on existing cavitation nuclei to produce sufficiently vigorous cavitation to mechanically subdivide tissue within the therapy volume. Therapy energy in the histotripsy process can be acoustic (e.g., ultrasonic). The transducer or transducers can be either single focus, or multi-focus, or phased arrays where the focus can be scanned in 1, 2, or 3-dimensions. The therapy transducer(s) can be contiguous spatially or can be separated spatially, using multiple windows into the therapy volume. The transducers can also operate at different frequencies individually or as an overall ensemble of therapy transducers. The therapy transducer(s) can also be mechanically scanned to generate larger therapy zones and/or a combination of mechanically and electronically (phased array) scans can be used. The therapy transducer(s) can also be used, as outlined herein, as sources of initiation and/or maintenance processes and procedures. The therapy transducer(s) can be intimately involved in the feedback processes and procedures as sources of interrogation sequences or as receivers (or even imagers). Thus, in some embodiments, the therapy pulses (or sequences) can initiate, maintain, and do therapy.

The multiplicity of transducers enables various embodiments where one of the therapy transducers could operate at a significantly lower frequency from the other(s). For example, the higher frequency transducer can initiate (predispose) and the lower frequency transducer can do the mechanical fractionation (dispose).

D. Feedback & Monitoring:

In some embodiments, feedback enables assessment of parameters related to noninvasive image guided therapy or drug delivery. The methods and devices depend on the fact that the actual therapeutic effect is the progressive mechanical subdivision of the tissue that can also provide enhanced drug transport (or other therapeutic or diagnostic effect) over one or more therapy pulses. Thus, the tissues exposed to the histotripsy process are changed physically. These physical changes are much more profound than changes produced by competing therapies. Furthermore, embodiments of the present teachings make it possible to monitor the therapeutic effectiveness both during and after the therapy process, which been unobtainable in previous noninvasive therapy procedures.

In some embodiments, feedback and monitoring can include monitoring changes in: backscatter from bubble clouds; speckle reduction in backscatter; ultrasound Doppler; acoustic emissions, as described below.

Backscatter from Bubble Clouds: This feedback method can determine immediately if the histotripsy process has been initiated, is being properly maintained, or even if it has been extinguished. The method also can provide feedback permitting the histotripsy process to be initiated at a higher intensity and maintained at a much lower intensity. For example, backscatter feedback can be monitored by any transducer or ultrasonic imager. By measuring feedback for the therapy transducer, an accessory transducer can send out interrogation pulses. Moreover, the nature of the feedback received can be used to adjust acoustic parameters (and associated system parameters) to optimize the drug delivery and/or tissue erosion process.

Backscatter, Speckle Reduction: Progressively mechanically subdivided thrombus results in changes in the size and distribution of acoustic scatter. At some point in the process, the scattering particle size and density is reduced to levels where little ultrasound is scattered, or the amount scattered is reduced significantly. This results in a significant reduction in speckle, which is the coherent constructive and destructive interference patterns of light and dark spots seen on images when coherent sources of illumination are used; in this case, ultrasound. After some treatment time, the speckle reduction results in a dark area in the therapy volume. Since the amount of speckle reduction is related to the amount of tissue subdivision, it can be related to the size of the remaining tissue fragments. When this size is reduced to sub-cellular levels, no cells are assumed to have survived. So, treatment can proceed until a desired speckle reduction level has been reached. Speckle is easily seen and evaluated on standard ultrasound imaging systems. Specialized transducers and systems can also be used to evaluate the backscatter changes.

Ultrasound Doppler: Thrombi partially or completely occlude the blood vessel, reducing or completely stopping the blood flow in the vessel. By breaking down the thrombus, the blood flow would be gradually restored, which can be monitored using ultrasound Doppler. Doppler measures the flow in the vessel downstream of the treatment location. Complete restoration of the blood flow is the indication of treatment completion.

Acoustic Emission: As a tissue volume is subdivided, its effect on microbubbles is changed. For example, bubbles may grow larger and have a different lifetime and collapse changing characteristics in intact versus fluidized tissue. Bubbles may also move and interact after tissue is subdivided producing larger bubbles or cooperative interaction among bubbles, all of which can result in changes in acoustic emission. These emissions can be heard during treatment and they change during treatment. Analysis of these changes, and their correlation to therapeutic efficacy, enables monitoring of the progress of therapy.

E. Acoustic Manipulation

In addition to breaking down thrombus, histotripsy has the ability to manipulate (e.g., trapping, moving, or rotating) an object in or near the focal location or target area in the presence or absence of a background flow (e.g., blood flow). In some embodiments, this manipulation is achieved using acoustic energy only, and therefore, is termed acoustic manipulation. It should be appreciated that in some embodiments acoustic manipulation can use other forces, such as background flow, or structure, such as vessel walls, to aid in the manipulation of the object.

To achieve acoustic trapping, a bubble cloud is generated in a tube-like structure (e.g., blood vessel) in the presence of a directional flow. As one or more bubbles expand and contract or collapses, alternating inward and outward fluid flow producing microstreaming. Activated by our short, intense histotripsy pulses, the collective expansion, contraction, and collapse of the bubble cloud occur extremely fast (on the order of microseconds), which result in significant fluid flow. It interacts with tube walls and causes a vortex-like flow that directs to the center of the bubble cloud. This fluid flow is maintained by histotripsy pulses and stops immediately when the histotripsy pulses end. When an object flows into the bubble cloud in the presence of the background flow, if the fluid flow rate is higher than the background flow, the object can be captured and trapped by the bubble cloud.

If the ultrasound pressure used to generate the bubble cloud is above the cavitation threshold to cavitate and damage the object, histotripsy can simultaneously fractionate and trap the object (e.g., blood clot).

To achieve acoustic moving, the bubble cloud location is moved by changing the position of the acoustic beam, which is realized by mechanically moving the therapy ultrasound transducer or electronically moving the transducer focus. Since the object is trapped in the bubble cloud, the object is moved with the cloud.

Rotational fluid flow may be created by a bubble cloud and its interaction with surrounding physical boundaries (e.g., liquid/solid boundaries). This flow results in rotation of the object in the bubble cloud, which is termed acoustic rotation.

Acoustic manipulation has clinical significance when applied to the thrombolysis application. For example, there is a concern that large clot fragments escape from the histotripsy thrombolysis treatment region and occlude the vessels causing hazardous embolization. Using acoustic trapping, a Non-invasive Embolus Trap (NET) is developed to prevent embolization caused by escaping clot fragments. The NET uses a secondary cavitating bubble cloud (or more bubble clouds) placed downstream of the treatment location, to capture and fractionate any clot fragments escaping the primary treatment cloud.

Acoustic manipulation also has a wider application outside thrombolysis. Acoustic manipulation can be used on an object other than a thrombus or thrombus fragment, e.g., bead, nano-particle, non-thrombotic emboli, arterial plaque, air bubbles, etc. For example, we can acoustically trap a bead encapsulating therapeutic agents such as pharmaceutics in a blood stream, acoustically move the bead to a treatment location (e.g., a tumor), delivery histotripsy treatment to fractionate the bead and release the therapeutic agents.

In connection with the present disclosure, the preliminary feasibility of the histotripsy thrombolysis technique was evaluated in a vessel model with static saline. The rate of thrombolysis versus pressure level was measured to assess efficiency. Cavitating bubble clouds were monitored using acoustic backscatter and correlated to the thrombolysis rate. Since circulatory flow in-vivo may have an effect on cavitation activity, the treatment was performed in a fast, pulsatile flow model. As histotripsy mechanically breaks down clots to debris particles, there is a concern that the debris may break off causing hazardous emboli that can occlude blood vessels and cause significant tissue ischemia with resultant morbidity and rarely mortality. To evaluate the risk of embolism, the sizes of clot debris generated by the procedure was measured. In addition, the use of a secondary cavitating bubble cloud as a non-invasive emboli filter was tested by capturing and further fractionating larger clot fragment.

Methods

A. Clot Formation

Fresh whole canine blood was obtained from research subjects and a citrate-phosphate-dextrose (CPD) solution (#C1765, Sigma-Aldrich Co., St. Louis, Mo.) was immediately added as an anti-coagulant at a ratio of 1 mL CPD per 9 mL blood. The blood was stored at 4° C. for up to three days prior to use. To induce clotting, a 0.5 M CaCl₂ standard solution (#21107, Sigma-Aldrich Co., St. Louis, Mo.) was mixed with the blood, using 0.05 mL CaCl₂ per 1 mL blood. The blood was drawn in 0.4 mL volumes into 1 mL syringes to form cylindrical clots with approximate dimensions of 4 mm (diameter)×20 mm (length). Syringes were transferred to a water bath with temperature 37° C. for 2 hours prior to the experiment to incubate the clots. All clots were then carefully removed from syringes, weighed, and transferred to a 0.9% room temperature (21° C.), air-saturated saline solution. All clots were treated within 6 hours of addition of CaCl₂. The resulting clots prior to treatment had a mean mass of 331+/−39.8 mg for those used in the static vessel model. Clots for the flow model were formed on a loose string by mounting the string longitudinally in the syringe. The string with the attached thrombus was removed after clotting, and the ends of the string were fixed to the tube. This technique was used to hold the clot in place under flow during the experiment.

B. In-Vitro Static Vessel Model

A stationary vessel model with no background fluid flow was employed for assessment of thrombolysis (FIG. 1 ). The model used a 6-mm diameter, 60-mm length low-density polyethylene (LDPE) tube with wall thickness of 500 μm to act as a vessel holding the clot. The LPDE plastic has an acoustic impedance similar to that of tissue. The tube was filled with 0.9% saline and the clot was carefully transferred to the tube. Tapered silicone rubber stoppers were used to plug the ends of the tube to contain the saline and clot debris from the treatment.

C. Ultrasound Generation And Treatment

The histotripsy treatment was performed using a piezocomposite 1-MHz focused transducer (Imasonic, S.A., Besancon, France) with a 15-cm focal length and 15-cm diameter. The focal volume is cigar-shaped, with dimensions 15 mm along the axis of propagation and 2.0 mm laterally at −3 dB peak negative pressure of 12 MPa. The therapy transducer has a 4-cm diameter hole in the middle for inserting an imaging probe. A class D amplifier used to drive the transducer. Ultrasound was pulsed using 5-cycle bursts at a pulse repetition frequency (PRF) of 1 kHz. Ultrasound was applied to clots at different peak negative pressures of 2, 4, 6, 8, 10, and 12 MPa with corresponding spatial peak pulse average intensities (IsPPA) of 150, 600, 2000, 3600, 5900, and 7000 W/cm². Pressure values for the ultrasound were obtained from waveforms recorded using a fiber optic probe hydrophone built in house. The probe was mounted with the fiber end facing perpendicular to the ultrasound propagation to prevent cavitation from corrupting measurements or damaging the tip. The signal was averaged over 200 pulses to reduce noise. Recorded signals are shown in FIG. 2 . No deconvolution was applied to the recorded waveforms.

All treatments were performed at room temperature (21° C.), in a degassed water tank with dimensions 100 cm×75 cm×67.5 cm. The transducer was mounted to a 3-axis motorized positioning system (Velmex, Inc., Bloomfield, N.Y.) controlled by a personal computer. The positioning system was used to position the clot in the transducer focus. Ultrasound was applied until the entire clot was dissolved or 300 seconds of treatment had occurred. The transducer focus was fixed throughout the treatment and the clot naturally moved into the focus until it was completely dissolved. The thrombolysis rate was calculated as the difference in initial mass and final mass of the clot divided by the amount of time ultrasound was applied (total treatment time).

D. Cavitation Monitoring Using Acoustic Backscatter

Acoustic backscatter from the cavitating bubble cloud was passively received using a 2.5-cm aperture 5-MHz focused single-element transducer with focal length of 10 cm (Valpey Fisher Corp., Hopkinton, Mass.). It was connected directly to an oscilloscope (Lecroy, Chestnut Ridge, N.Y.) for data collection. The backscatter signal was recorded by the oscilloscope every 300 ms in a 20 μs window timed to capture the scattered therapy pulse. Tissue fractionation only occurs when initiation of a temporally changing acoustic backscatter is detected corresponding to formation of a cavitating bubble cloud. Here the initiation of the temporally changing scattered wave was detected. The backscatter receiver was positioned facing 90° from the therapy transducer instead of through the central hole of the therapy transducer, since the hole was occupied by an imaging probe. This technique measures the continuous dynamic change in scattering energy due to pulse-to-pulse changes in the bubble cloud. Briefly, the normalized energy for each backscatter waveform is calculated. A moving standard deviation over time of the normalized energy is then calculated. When this standard deviation (pulse-to-pulse variation in backscatter) is above a set threshold for 3 or more consecutive points, initiation of a bubble cloud occurs. It should be understood that other predetermined thresholds can be established to quantify the initiation of the bubble cloud. From this, the total amount of time a bubble cloud was present during treatment for each trial could be calculated. The initiation threshold for each pressure level was determined by linear extrapolation from measurements at the lowest pressure levels, where no initiation was observed.

E. Ultrasound Imaging Feedback

A 5-MHz ultrasound imager (System FiVe, General Electric, U.S.A) was used for targeting the clot and monitoring treatment progress. The imager was positioned through the central hole in the therapy transducer such that it always imaged the therapy plane. For targeting prior to treatment, a bubble cloud was generated at the focus of the transducer in the empty water bath and appeared as a hyperechoic zone, which refers to a region with increased amplitude on an ultrasound image. The position of the hyperechoic zone was marked on the image as the focus. Once the tube containing the clot was added to the water bath, the therapy transducer was positioned so that the focus marker was aligned at one end of the clot. Once the targeting is achieved, histotripsy treatment was applied to the clot. The treatment progress and completion was monitored through reduced echogenicity on the B-Mode image resulting from breakup of the clot.

F. Measurement of Histotripsy Clot Debris

There is a concern that the clot fragments or debris generated by histotripsy may form emboli and occlude downstream vessels. To address this issue, the suspended clot debris was serially filtered through 1 mm, 100 μm, 20 μm, and 5 μm filters after treatment to measure the total weight of particles in each size category. The dry weight of each filter was measured prior to treatment. After filtering, the samples were dried over 12 hours, and each filter was reweighed.

To obtain a more sensitive measurement of particle distribution, the suspended clot debris from the stationary vessel model was also measured using a particle sizing system, a Coulter Counter (Multisizer 3, Beckman Coulter, Fullerton, Calif.). After treatment, the clot debris saline suspension was collected from each of the treated clots and the debris size distribution was measured using the Coulter Counter. This device measures the impedance change due to the displacement by the particle volume of a conducting liquid in which the particles are suspended. The impedance change is proportional to the particle volume. Volume of debris particle is calculated and diameter is estimated assuming a spherical shape for each particle. The measurement size range is 2-60% of the size of aperture tube which is part of the Coulter Counter. A 100-μm diameter aperture tube was used to achieve a dynamic range of 2-60 μm in diameter. Debris larger than 60 μm which blocked the aperture tube caused interruption of the measurement, and was noted. The sizing resolution is approximately 1% of the particle diameter. Two measurements were taken for each sample.

G. Thrombolysis in a Pulsatile Flow Model

To test the effect of high flow rates on histotripsy thrombolysis, clots were treated in a circulatory model with filtered water (FIG. 1 ). The flow model used a pulsatile flow pump (Harvard Apparatus Pulsatile Blood Pump, Holliston, Mass.) with settings to control the pulses per minute and stroke volume. The pump was attached with vinyl tubing to one end of the vessel-mimicking LPDE tube in a water bath to allow flow into the tube. 1-mm and 100-μm rated filter paper was placed downstream from the tube to capture large clot debris and fragments. The pulsatile pump was set to operate at 70 beats per minute (bpm) with a stroke volume of 15 mL and a systolic to diastolic ratio of 35:65. These values were chosen to produce a mean flow velocity of 50 cm/sec in the 6 mm diameter LPDE tube, which is an upper limit for mean blood flow velocities typically found in major vessels.

Clots were formed on a string, as previously described. Both ends of the string were secured to hold the clot in position under flow. The transducer focus was scanned along the clot in the direction opposite of flow at a rate of 0.1 mm/s. After treatment, any remaining clot was removed from the tube and weighed to calculate the thrombolysis rate.

Results

A total of 56 clots were treated in the stationary model. At peak negative pressures (p−) of 2 and 4 MPa, no visible clot disruption was observed. At p− of 6 and 8 MPa, the clot was partially fractionated into tiny debris after 300 seconds of histotripsy treatment. At p− of 10 and 12 MPa, the entire clot was always completely fractionated within 300 seconds of treatment. Clot disruption was only observed visually when a bubble cloud was initiated at the focus of the transducer. If the bubble cloud was generated adjacent to the clot (within 10 mm), the clot would naturally move towards the bubble cloud until the center of the clot was aligned with the bubble cloud. During thrombolysis, the color of the clot changed from red to white at the surface where it was eroded, and then further dissolved until no visible fragments remained. This suggests red blood cells were destroyed prior to breakdown of the extracellular clot matrix. The progression of a treatment is shown in FIG. 3 .

Section A reports the change in thrombolysis rate with acoustic pressure. Thrombolysis rate is also correlated with acoustic backscatter in section B, which reports the initiation detection of a cavitating bubble cloud at different pressures. Further, Section C describes how treatment was monitored using imaging feedback. Section D reports the size distribution of debris generated during thrombolysis. Section E shows results from performing histotripsy thrombolysis under fast circulatory flow. Finally, Section F demonstrates the ability of histotripsy to effectively trap free clot particles and further fragment them.

A. Thrombolysis Rates Versus Pressure

The thrombolysis rate was plotted as a function of peak negative pressure (p−=0 to 12 MPa) in FIG. 4A (mean and standard deviation, n=8). The corresponding peak positive pressure and ISPPA are listed in Table 1.

TABLE 1 Number of trials with bubble cloud initiation and significant thrombolysis at each pressure level. (n = 8 at each pressure) Clot Clot Weight Weight # Trials # Trials P− P+ Isppa Istpa (Pre) (Post) with with (Mpa) (Mpa) (W/cm²) (W/cm²) (mg) (mg) Thrombolysis* Initiation 0 0 0 0 340 ± 38 300 ± 40 0 0 2 3 150 1 320 ± 54 285 ± 54 0 0 4 8 600 4 342 ± 34 296 ± 27 0 0 6 20 2000 14 316 ± 39 251 ± 73 4 7 8 32 3600 25.2 354 ± 25  64 ± 52 8 8 10 39 5900 41 310 ± 41  1.2 ± 3.5 8 8 12 43 7000 49 332 ± 32  1.2 ± 3.5 8 8

In the control group (p−=0 MPa), clots were placed in saline for 5 minutes without ultrasound exposure, and visible clot disruption was never observed. Similarly, at p− of 2 and 4 MPa, no visible changes were observed during treatment and the thrombolysis rate was not statistically different from that of the control group. The thrombolysis rate was 0.13+/−0.038 mg/sec for the control group and 0.12+/−0.047 mg/sec at pressure of 4 MPa (t-test, P=0.22). It is possible that most of the weight reduction for each of these three groups was due to handling of the clot to transfer it into and out of the tube or dissolution of clot serum into the saline.

At p−=6 MPa, 4 of 8 clots treated had rates similar to the control group (0.066+/−0.047 mg/sec). The other 4 clots had significantly higher thrombolysis rates (0.366+/−0.087 mg/sec) than control. At p−≥8 MPa, a significant increase in thrombolysis rate was observed for all clots in comparison to the control group (paired t-test, P<0.0001). At the highest pressures (p− of 10 and 12 MPa), all clots were completely fractionated in times between 80-260 seconds. There was an increase in thrombolysis rate with peak negative pressure between 6-12 MPa (t-test, P<0.05). The mean rate was 0.21+/−0.17 mg/sec at p− of 6 MPa and 2.20+/−0.85 mg/sec at p− of 12 MPa.

B. Cavitation Monitoring Using Acoustic Backscatter

Detection of temporally changing acoustic backscatter was used to monitor a cavitating bubble cloud. Without the initiation and maintenance of this temporally changing backscatter, no tissue fractionation was generated by histotripsy. It was determined that without initiation, no thrombolysis was observed, i.e., the thrombolysis rate was similar to the control rate. In 28 of 31 treatments (90%) where initiation was detected, the thrombolysis rate was significantly higher than the control. Table 1 shows the number of events for each pressure where thrombolysis occurred, as well as the number of events where initiation occurred. For purposes of discussion, thrombolysis was considered to have occurred when the thrombolysis rate was greater than twice the control rate.

The percentage of time a bubble cloud was initiated throughout treatment was calculated. The percentage of initiated time is the amount of time that temporally changing acoustic backscatter is detected divided by the total treatment time. The percentage of initiated time was plotted as a function of peak negative pressure (FIG. 4B). P− of 2-4 MPa had very low mean values for percentage of initiated time (<0.5%) and thrombolysis was never observed at these pressure levels. P− of 6 MPa had an intermediate percentage of initiated time of 56%. At this value, 4 clots where thrombolysis occurred also had a high percentage of initiated time (mean 87%) versus 4 with low thrombolysis rates (mean 25%). For 8-12 MPa, the mean percentage of initiated time was >99.6% and thrombolysis always occurred. This supports the claim that the cavitation cloud is necessary for histotripsy thrombolysis.

The thrombolysis rate at P−=6 MPa was previously defined as the mass loss divided by the total treatment time. However, it was shown that during only 56% of the treatment time was a bubble cloud present. To obtain an estimate of the thrombolysis rate only when a cloud is initiated, the total initiated time can be used to calculate rate instead of total treatment time. This calculation gives a thrombolysis rate of 0.58+/−0.17 mg/sec, which is significantly higher than the thrombolysis rate calculated using the treatment time. Since thrombolysis appears to only occur when the bubble cloud is initiated, this rate provides a better measure for the efficiency of the bubble cloud.

C. Ultrasound Imaging

The histotripsy thrombolysis treatment was monitored with B-mode ultrasound imaging in real-time. Prior to application of ultrasound, the clot appeared as a hyperechoic zone inside the tube walls on the B-mode ultrasound image (FIG. 5A). During the treatment, a bubble cloud was generated in the tube adjacent to the clot, which appeared as a temporally changing hyperechoic zone at the therapy transducer focus (FIG. 5B). Interference of the therapy acoustic pulses with the imager caused only minimal corruption of the image due to the low duty cycle used for treatment (0.5%). As the treatment progressed, the clot's hyperechoic zone reduced in size and echogenicity. The bubble cloud remained on the clot surface throughout the treatment. Once the clot was entirely fractionated, its hyperechoic zone on the image disappeared and the inside of the tube became hypoechoic (FIG. 5C).

D. Measurement of Histotripsy Clot Debris

To obtain the size distribution of clot debris generated by histotripsy, samples were measured using filter papers rated to 5 μm, 20 μm, 100 μm and 1 mm. The wet and dry weights of several whole clots were recorded. Whole clots with a wet weight of 350 mg were reduced to 100 mg weight once dried. The change in dry weight of the filter was measured to estimate the debris size distribution. All four filters' dry weights changed by ≤1 mg. No significant difference was found between control and any of the treated samples. These results suggest that at least 96% (96 mg of 100 mg) of the clot was broken down to particles smaller than 5 μm.

Additionally, saline samples containing suspended clot debris were removed from the tube after each treatment and measured by the Coulter Counter. The mean debris distributions between 2-60 μm particle diameter are shown in FIG. 6 . For control clots, a mean of 95%+/−4% of the debris volume was between 2-10 μm, 3% between 10-30 μm, and 2% between 30-60 μm. In treatment samples where thrombolysis was detected, 72-94% of the debris was 2-10 μm, and 3-12% was between 30-60 μm. The mean number of particles counted in the treatment samples was similar to the controls. Samples treated at the highest pressures (10 and 12 MPa) had a higher percentage of larger particles (30-60 μm) than lower pressures. Debris distributions also showed a large increase in particles smaller than 6 μm for those treated at high pressures, suggesting the disruption of individual cells.

In 2 of 56 measurements (two measurements per treatment) where thrombolysis was not detected, the 100 μm tube was blocked. In 9 of 56 measurements where thrombolysis was detected, the 100 μm tube was blocked. The blockage of the tube suggested the presence of one or more particles larger than 60 These results suggest that particles larger than 60 μm are generated during the treatment, although some of them may result from process other than histotripsy thrombolysis.

E. Thrombolysis Under Flow

Since cavitation may be influenced by the presence of flow, e.g., cavitation nuclei may be swept away, the feasibility of histotripsy thrombolysis was also tested in a fast flow environment. Clots were treated under a mean flow velocity of 50 cm/s. This value is the upper limit of mean flow velocities in major vessels. Clots formed for this experiment were smaller (150+/−26 mg) than those used in the stationary clot model due to difficulty forming large clots on the string. Eight clots were treated at p−=12 MPa, and clot weight was reduced by 72%+/−21% (mean and standard deviation) in the fast flow in 100 seconds. During this time, the therapy focus was scanned to cover the entire clot at a scanning rate of 0.1 mm/sec. The thrombolysis rate was 1.07.+/−0.34 mg/s, which is significantly higher than the control rate of 0.27+/−0.12 (t-test, P<0.0002). However, the rate at p−=12 MPa here was lower than those in static saline at the same pressure level.

Serial filters of 1 mm and 100 μm were used to capture any large clot debris or fragments generated by histotripsy treatment. No measurable debris was captured by the 1 mm filter. In two of the eight treated clots, 5% and 12% of the initial clot weight was captured by the 100 μm filter paper. In one of eight control clots, 17% of the clot weight was captured by the 100 μm filter. All other filters showed less than 3% variance in weight before and after the experiment.

F. Acoustic Manipulations

The acoustic manipulations have been demonstrated the in vitro experiments. For example, preliminary results show that when a clot fragment flows into the cavitating bubble cloud generated by histotripsy in a vessel tube, it can be stopped (and trapped) near the cloud and further fractionated into small debris. Clot fragments of diameter 3 mm were cut from formed clots, and injected into the circulatory model with a background flow of −5 cm/s and upstream from the transducer focus. In the example shown in FIG. 7 , a bubble cloud was generated in the tube center using p− of 12 MPa. The bubble cloud occupied approximately ⅓ of the vessel tube diameter. The 3 mm clot fragment drifted into the bubble cloud and became trapped near the transducer focus. While trapped in the cloud, the clot was further fractionated. Within one minute from when the clot fragment entered the bubble cloud, it was completely broken down with no visible fragments remaining.

This experiment was repeated 13 times to test the ability of histotripsy to capture clot fragments that would potentially be hazardous emboli. Of the 13 trials, all clots were stopped as they drifted into the bubble cloud. The clot fragments were further fractionated to smaller particles which were then ejected from the cloud. The largest particles ejected from the cloud were sub-millimeter. When the clot fragments were captured, 7 of the 13 clots were completely fractionated in a time of 142+/−99 seconds. 5 of 13 clots were partially fractionated before being swept out of the tube. They were held near the bubble cloud for a mean time of 132+/−66 seconds. 1 of 13 clots was held near the bubble cloud for 5 seconds, but was then swept downstream by background flow and remained unfragmented.

Discussion

Current clinical thrombolysis methods, including catheter-based procedures and thrombolytic drugs, have major drawbacks. Both these methods can cause severe bleeding and catheters are invasive and can cause infection. Ultrasound-enhanced thrombolysis may increase the reperfusion rate, but can also cause bleeding, as it involves the use of thrombolytic drugs. Histotripsy does not require drugs and is non-invasive, and thus has the potential to overcome these limitations. In addition, results show that histotripsy can dissolve 300 mg clots in 1.5-5 minutes. The thrombolysis rates demonstrated from in-vitro experiments are order of magnitude faster than those for drugs. Since histotripsy is non-invasive and does not involve a complex procedure to insert catheter into the treatment region, it would also require less time and lower cost than a surgical catheter.

In connection with the present disclosure, ultrasound by itself was applied to cause thrombolysis. Previous researchers explored the use of high-intensity focused ultrasound alone to break down blood clots. Rosenschein suggested that cavitation collapses were the underlying cause of damage. Cavitation has been and is still generally regarded as uncontrollable and unpredictable. The mechanism of cavitation has been studied and found that it can be well controlled using specific ultrasound pulse sequences to produce targeted fractionation of soft tissue including blood clots. A histotripsy pulse sequence includes very short pulses (<50 μs) at very high pressures (>6 MPa) and low duty cycles (0.1-5%). Our hypothesis regarding the mechanism of histotripsy is that each ultrasound pulse creates a cluster of microbubbles localized at the transducer focus. The microbubbles within the cluster collapse causing local stresses which remove a portion of the targeted tissue. These individual microbubbles also act as nuclei which can be excited by subsequent pulses, predisposing tissue in the focal region to further damage. It has been found that tissue fractionation only occurs with the initiation and maintenance of a cavitating bubble cloud, which can be achieved using appropriate ultrasound pulse sequences (i.e., histotripsy pulses). Bubble cloud initiation and maintenance can be detected by cavitation feedback monitoring. Cavitation feedback includes ultrasound imaging and acoustic backscatter signals with specific traits, such as high temporally-changing backscatter amplitudes and increased broadband noise levels.

Accordingly, our understanding of histotripsy is consistent with the results from the present disclosure. It has thus been found that thrombolysis only occurs when the cavitating bubble cloud is detected by acoustic backscatter. The acoustic parameters effective for thrombolysis are also consistent with the parameters found effective for other soft tissue fractionation, using short pulses, a low duty cycle, and a peak negative pressure >=6 MPa. While a correlation between the cavitating bubble cloud and the fractionation of tissue has been demonstrated, how individual bubbles interact with the targeted tissue to cause fractionation is not sufficiently understood. A variety of damage mechanisms have been proposed, including collapse of individual microbubbles, bubble cloud collapse, microstreaming-induced shear forces and acoustic streaming, or combinations of these effects.

One major advantage of histotripsy is that it can be easily guided by real-time ultrasound imaging for targeting and treatment monitoring. The results suggest that histotripsy thrombolysis can be also guided using real-time ultrasound imaging. The bubble cloud is highly echogenic and dynamic on a B-mode image, and blood clots can be readily identified and aligned to the therapy focus. The progression of thrombolysis can also be monitored by observing clot echogenecity and Doppler color flow mapping of the occluded vessel. Using these techniques, histotripsy thrombolysis can be visualized and guided by real-time ultrasound imaging feedback, which is a primary challenge for any non-invasive technique and essential to ensure the treatment accuracy and efficiency.

As bubble dynamics are highly dependent on their environment, there is a possibility that the effects of histotripsy may be hindered by high blood flow velocities. The maintenance of a bubble cloud likely depends on previously initiated nuclei, and those nuclei may be swept out of the focus by background flow. The feasibility of histotripsy thrombolysis at the highest natural flow velocity in-vivo (50 cm/sec) was studied. When clots were subjected to a high-velocity pulsatile flow, histotripsy was still capable of fractionating the clot. This result shows that a cavitation cloud can be initiated and maintained in the fast flow. In this situation, the thrombolysis rate was lower than those treated without flow. This could be because the clot is held in a fixed position in the flow model, and the transducer focus must scan along the clot to completely fractionate it. Since the scanning velocity may not have been optimized, some of the clot remained intact after treatment in several cases.

Since histotripsy causes damage by microbubbles that are very small (particularly when they collapse), histotripsy can fractionate tissue to tiny debris. When histotripsy is used to treat soft tissues (e.g., kidney, myocardium, and prostate), it fractionates tissue to a sub-cellular level with debris of a few microns or smaller. Similarly, histotripsy can fractionate a blood clot into small debris. The filter measurements suggest >96% of the debris weight was smaller than 5 μm. The Coulter Counter method also showed that small particles (2-10 μm) were a majority (74-94%) of debris in the range of 2-60 μm. The fact that the number of particles counted in both control and treated samples was similar suggests that a majority of debris generated by histotripsy is outside of the Coulter Counter range (i.e., likely smaller than 2 μm). Both the filter and Coulter Counter measurements indicated that histotripsy breaks down the clot below the size of individual red blood cells (6-8 μm). 100 μm mechanical filters have been used to successfully prevent embolism, and only particles larger than this may be considered potentially unsafe emboli. The Coulter Counter measurement suggests that there are occasionally debris particles larger than 100 μm. Debris generated at lower pressures (6 and 8 MPa) also contained fewer large fragments than higher pressures. It is possible that the acoustic parameters could be adjusted to minimize the number of large particles. However, it is not clear that whether particles >100 μm can be avoided completely during treatment.

A method to reduce the risk of embolism was tested, using a bubble cloud to capture and fractionate the emboli. In the preliminary test, the bubble cloud could be used to trap a large clot particle near the focus and further fragment it. This acoustic trapping ability is likely due to cavitation-induced fluid flow. Microstreaming can generate a flow pattern pulling particles towards a single bubble even in the presence of an overall directional flow. This phenomenon is also applicable (and may be magnified) when bubbles act collectively as a cloud. Using the acoustic trapping property of histotripsy, development of a Non-invasive Embolus Trap (NET) is anticipated, which is a secondary cavitating bubble cloud set downstream of the primary treatment cloud to capture and further fractionate any escaping clot fragments. The NET could be created by a separate transducer and effectively act as a filter for large emboli. Preliminary tests indicated that clot fragments can be trapped and further broken down into smaller fragments. In some embodiments, the observed bubble cloud was only ⅓ of the tube diameter and did not occupy the whole tube. Its small size is possibly why some fragments escaped from the cloud before complete fractionation and a significant amount of debris larger than 100 um were measured. By applying appropriate acoustic parameters, the bubble cloud size can be changed to occupy a larger portion of the tube and maintain greater control over particles. It is possible that different sets of parameters will be optimal for the NET than for the thrombolysis treatment. The NET would add an additionally degree of safety to the treatment, and may be an effective means to prevent embolism in other procedures that may be associated with the risk of embolization.

Aside from embolism, there are other concerns that must be addressed regarding the safety of histotripsy thrombolysis. As histotripsy mechanically fractionates a clot, there is a possibility that the process might also damage the surrounding blood vessel. As part of the present disclosure, clots were treated in a canine aorta segment and vena cava segment using the same acoustic parameters as discussed herein at a pressure level of p−=12 MPa. FIG. 8 shows the histology of control and treated segments after 300 seconds of exposure. Histotripsy-treated aorta and vena cava walls remained intact in initial histological studies. The vessel's higher resistance to histotripsy-induced damage is likely due to its mechanical ductility being higher than that of soft tissues. In addition to mechanical damage, the vessel may also be damaged by ultrasound-induced heating. However, histotripsy uses a very low duty cycle and the time-averaged intensity at the focus is lower than that required to cause thermal necrosis.

Hemolysis may also be an adverse effect of histotripsy thrombolysis. Red blood cells are easily damaged by shear forces, and have been previously shown to be susceptible to cavitation. The debris measurements suggest that histotripsy breaks down red blood cells within the clot to subcellular fragments. Therefore, it is also likely that free erythrocytes in blood will also be lysed. When hemolysis occurs in a significant volume of blood, it can cause hemolytic anemia and hyperkalemia. As the treatment is only localized to the small focal volume and the flow rates in occluded vessels are generally low, it is unlikely that large volumes of blood will be lysed during the treatment. Ultrasound (and cavitation in particular) has also been observed to cause platelet aggregation and activation, which facilitates clotting. There is a possibility that histotripsy may cause clot reformation by activating platelets.

CONCLUSION

The results show that histotripsy mechanically fractionates blood clots into small particles. Histotripsy can completely remove large clots in both a controlled static saline environment and a fast flow model simulating in vivo blood flow. In both cases, the treatment time lasted less than five minutes for large clot (140-300 mg). Thrombolysis only occurred when the presence of a dense cavitation cloud was detected. Debris particles generated by histotripsy thrombolysis was measured and revealed >96% particle weight smaller than 5 μm, although some particles >100 μm were generated. To address this issue, the ability of histotripsy to trap and further fractionate large clot fragments was tested. It was found that the cavitating bubble cloud can capture and simultaneously fractionate a clot fragment flowing through the cloud. This ability may provide a novel tool to capture and eliminate hazardous emboli by setting a secondary bubble cloud downstream of the treatment cloud. These findings suggest that histotripsy is a viable new thrombolysis strategy.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention. 

What is claimed is:
 1. A method for controlled mechanical fractionation of a target tissue, comprising: outputting an ultrasound pulse sequence from a transducer array resulting in cavitation forming a bubble cloud; detecting a location of the bubble cloud; actuating the transducer array such that the bubble cloud is spatially positioned within the target tissue in response to the location of the bubble cloud; monitoring backscatter from the bubble cloud with the transducer array; and adjusting one or more parameters of the ultrasound pulse sequence based on the monitored backscatter from the bubble cloud.
 2. The method of claim 1, wherein monitoring backscatter further comprises monitoring backscatter from the bubble cloud with a subset of transducer element(s) of the transducer array.
 3. The method of claim 1, wherein an absence of backscatter indicates an extinguished bubble cloud.
 4. The method of claim 1, wherein adjusting the one or more parameters further comprises reducing an intensity of the ultrasound pulse sequence.
 5. The method of claim 1, wherein adjusting the one or more parameters further comprises increasing an intensity of the ultrasound pulse sequence.
 6. The method of claim 1, wherein adjusting the one or more parameters further comprises decreasing an intensity of the ultrasound pulse sequence to an intermediate intensity that is below a value that would not otherwise form a bubble cloud. 